Proportional-delayed zero-crossing frequency-drift estimator devices are provided. Given N-number of time samples, the current zero-crossing time estimation technique uses only two zero-crossing time samples for repetition-rate estimation. The term "zero-crossing point" refers to the point where a sinusoidal waveform varies from a positive to a negative value and crosses the zero value in the process. The proportional-delayed zero-crossing frequency-drift estimator devices employ four zero-crossing time samples by utilizing both proportional zero-crossing points for current information and delayed zero-crossing points for past information so that the noises in the time samples will be smoothed out making a less noisy estimation. The proportional-delayed zero-crossing frequency-drift estimators are composed of a hysteretic nonlinear converter, a zero-crossing time-difference counter, a group of shift registers and adders, a repetition-rate algorithm, a reciprocal operator and a means for differentiation operation. A number of different embodiments are provided, but all embodiments include a hysteretic nonlinear converter which enhances the zero-crossing features and blocks the signal random noise so that the zero-crossing point is measured more accurately.